Aprilaire 700M Whole House Fan Powered Humidifier, Single Sensor High Output Furnace Humidifier BUY WITH CONFIDENCE This humidifier was designed and manufactured in the U.S.A. by Aprilaire  the inventor of the whole home evaporative humidifier, and the leader in indoor air quality solutions BUILT-IN FAN pulls heated air directly from the furnace through the humidifier FULL COVERAGE up to 4,200 square feet in tightly built homes with a capacity of 18 gallons per day SINGLE SENSOR built into a standard control monitors indoor relative humidity for use in manually setting the percent humidity in your home HUMIDITY FOR HEALTH Aprilaire Humidifiers can help you maintain optimal humidity in your home of 35%  45% which has been shown to reduce the incidence of respiratory infections and symptoms related to allergies and asthma by minimizing the formation of bacteria and viruses, fungi, and dust mites.